MULTIPLE ORIENTATIONS AND GENERATIONS OF BASAL CREVASSE FILLS, MATANUSKA GLACIER, ALASKA

Young (post 1952) laminated, silt-rich debris bands are a common feature in the englacial ice near the terminus of the Matanuska Glacier, Alaska. These tritium-enriched planar debris bands are hypothesized to be the result of injection of supercooled waters into basal crevasses (Ensminger and others, 1999). While their formation and relationship to the basal ice and the subglacial drainage system has been researched, a study of the orientation of these basal crevasse fills previously has not been attempted. Strike, dip, and GPS measurements were taken during the summer 2000 melt season on a series of laminated debris bands in an overdeepening near the glacier terminus. Geographic Information Systems, rose diagrams, and aerial photography were used to plot and analyze orientations.

The most distinctive basal crevasse fills in the study area strike generally east-west, which is approximately parallel to local ice flow. However, some debris bands strike north-south and in other orientations. Basal crevasse fills most commonly dip steeply south. Several newly formed crevasses crosscut the older laminated debris bands and have similar north-south orientations to one another. These crevasses are interpreted to be basal in origin because they were blind to the surface along portions of their lengths. Additional observations suggesting a basal origin include that these crevasses had glaciohydraulically supercooled, silt-laden water upwelling within them. Also, frazil ice, evidence of glaciohydraulically supercooled water, grew inward from the crevasse walls. Therefore, we hypothesize that these basal crevasses are newly forming laminated debris bands (Ensminger and others, 1999). Orientation analyses conclude that east-west trending, tritium-enriched basal crevasse fills are crosscut by the new north-south trending crevasses. Several orientations of the basal-crevasse-fill type laminated debris bands exist, suggesting multiple generations for their formation, which can be dated relative to one another.